1. Field of the Invention
The invention relates to a method of treating residues after purification of fumes from incinerated household rubbish and/or industrial waste.
2. Descrition of the Related Art
Incineration plants convert household rubbish into clinker together with dust-laden fumes containing noxious gases. The fumes are generally purified with lime, thus capturing hydrogen chloride and sulphur-containing gases and converting them into chloride of lime and calcium sulphate, whereas heavy metals in the fumes are adsorbed on the surface of the lime particles. The products obtained at the filter outlet after the said treatment with lime together constitute the residues, which are at present discharged or stored at places conforming to national, European or international regulations.
These residues accordingly comprise products resulting from reaction with lime, adsorbed amphoteric heavy metals, calcium carbonate, lime and products resulting from incineration and captured by treatment with lime and in the form of inert dust (for example silica and alumina).
The composition or various constituents of these residues depends of course on the nature of the incinerated rubbish and waste and on the method of treatment of the fumes.
With regard to pollution, the following three categories of residues are distinguished:
substances free from polluting products;
substances containing noxious or polluting products but in stable form, that is without risk to the external environment, and
substances containing harmful elements which are transferable to the external environment; these make up about 2% by weight of the residues.
As the skilled man will understand, this latter category is a potential source of pollution. It consists inter alia of heavy metals which are accessible and therefore capable of contaminating the external environment, and gives rise to present regulations requiring storage at suitable places.
One aim of the invention therefore is to provide a process for treating the residues after purification of fumes from incinerated household rubbish and or industrial waste so as to extract heavy metals and recycle the maximum proportion of reagents.
Another aim of the invention is to provide a said process for also separating the various elements in the residues in order to make optimum use of them, recycle them or dispose of them in accordance with the regulations in force.
An additional aim is to provide a method of this kind for using carbon dioxide emitted for example by incineration plants.
Another aim of the invention is to provide a said method for making optimum use of the products obtained, at minimum cost.
These aims, together with others which will appear hereinafter, are addressed by a method of processing residues after purification of fumes from incinerated household rubbish and/or industrial waste in accordance with the invention and characterized in that it comprises the following steps:
Axe2x80x94the residues are desalted by washing with sodium carbonate in sufficient quantity or slightly In excess for solubilising all the salts including the sulphates and for precipitating the soluble calcium, the pH being above 11, followed by separation of solids from liquids to obtain a desalinated cake;
Bxe2x80x94the cake from step A is leached with sodium carbonate in sufficient quantity or slightly in excess for reacting with the lime present in the cake, or added if necessary, to obtain the alkalinity necessary for solubilisation of amphoteric metals, the pH being approximately 14, the solids being separated from liquids so as to obtain a cake and a superatant which is neutralised with carbon dioxide, and the recovered solution is filtered so as to obtain metal hydroxide sludge and an aqueous solution of sodium carbonate which can be recycled to the said step B, and
Cxe2x80x94the residues from step B are neutralised at a PH between 6.5 and 8.5.
Advantageously, step A is preceded by a step D of prewashing the residues with water in order to eliminate soluble calcium salts, particularly calcium chloride.
Preferably the solution from step A is neutralised by blowing in carbon dioxide and the liquids are separated from solids to obtain metal hydroxide sludge and a neutralised solution. The metal hydroxide sludge and a neutralised solution are treated in the same way. The same is done with the solution in step D. The neutralised solutions are mixed and filtered to obtain a supernatant comprising calcium chloride and sodium chloride brine and precipitated calcium sulphate or gypsum.
Advantageously a step E of washing the filter cake from step B is inserted between steps B and C in order to eliminate the maximum proportion of soda and metals remaining in the interstitial liquid. The washing solution is preferably treated by injection of carbon dioxide until the pH approaches 11. The sodium carbonate solution is advantageously recycled to step A.
The step C of neutralising the residues from step B or E is effected by injection of carbon dioxide in order to precipitate the calcium carbonates and obtain purified residues.